Hydrogen is utilized in a wide variety of industries ranging from aerospace to food production to oil and gas production and refining. Hydrogen is used in these industries as a propellant, an atmosphere, a carrier gas, a diluent gas, a fuel component for combustion reactions, a fuel for fuel cells, as well as a reducing agent in numerous chemical reactions and processes. In addition, hydrogen is being considered as an alternative fuel for power generation because it is renewable, abundant, efficient, and unlike other alternatives, produces zero emissions. While there is wide-spread consumption of hydrogen and great potential for even more, a disadvantage which inhibits further increases in hydrogen consumption is the absence of an infrastructure that can provide generation, storage and widespread distribution of hydrogen.
One way to overcome this difficulty is through distributed generation of hydrogen, such as through the use of fuel processors to convert hydrocarbon-based fuels to hydrogen-rich reformate. Fuel reforming processes, such as steam reforming, partial oxidation, and autothermal reforming, can be used to convert hydrocarbon-based fuels such as natural gas, LPG, gasoline, and diesel, into hydrogen-rich reformate at the site where the hydrogen is needed. However, in addition to the desired hydrogen product, fuel reformers typically produce undesirable impurities that reduce the value of the reformate product. For instance, in a conventional steam reforming process, a hydrocarbon feed, such as methane, natural gas, propane, gasoline, naphtha, or diesel, is vaporized, mixed with steam, and passed over a steam reforming catalyst. The majority of the hydrocarbon feed is converted to a reformate mixture of hydrogen and impurities such as carbon monoxide and carbon dioxide. To reduce the carbon monoxide content, the reformate is typically subjected to a water-gas shift reaction wherein the carbon monoxide is reacted with steam to form carbon dioxide and hydrogen. After the shift reaction(s), additional purification steps may be utilized to bring the hydrogen purity to acceptable levels. These purification steps can include, but are not limited to, methanation, selective oxidation reactions, membrane separation techniques, and selective adsorption such as temperature swing adsorption and/or pressure swing adsorption processes.
Gas separation by pressure swing adsorption (PSA) is achieved by coordinated pressure cycling over an adsorbent bed that preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. In a conventional PSA device, two or more adsorbent beds are connected in alternating sequence by directional valving to pressure sources and sinks for establishing the changes of working pressure and flow direction. In another conventional PSA device, flows to and from adsorbent beds are controlled by a rotary distribution valve that is rotated to cycle the adsorbent beds through adsorption and regeneration phases. For instance, the separation of oxygen from air is a known application of such conventional PSA devices. However, in such applications the composition of the gas mixture, its pressure and/or flow rate are typically fixed and known. In contrast, the integration of a PSA device to a fuel processor that produces a reformate of varying composition, pressure and/or flow rate imposes challenges to the efficient operation of such an integrated system.